Retrofit of hundreds of Danish turbines

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Two different fixes for unacceptable levels of wear on the teeth of gears fitted in large wind turbines less than two years old are being carried out by Denmark's two biggest wind turbine manufacturers, Vestas and NEG Micon. The extraordinary wear, now recognised as a series fault, was discovered in gear boxes from three suppliers to the wind industry and came to light during routine inspections (Windpower Monthly, December 1998). Gear boxes in over one thousand Vestas and NEG Micon wind turbines are affected.

The more major retrofit is being carried out by Vestas. It will replace gearboxes or strengthen bearings on all its turbines rated at 600 kW, a total of 880 units, at a cost of DKK 24 million (EUR 3.2 million). The aim is to prevent bearing or gear box damage in the future. Faults have so far been found on 40 gear boxes and 60 bearings. Of the 880 machines, 227 are in Denmark, 158 in Spain, 264 in Germany, 28 in Sweden, and 24 in China, with the rest spread over a number of countries.

Vestas' service manager Hans Laurids Pedersen says all the gear boxes involved are made by Valmet of Finland, although having tested five different types of oil and three makes of gear box, Vestas says the faults occur whatever the combination. Mystery still surrounds the cause. "All the experts agree that the bearings will have a longer life than the wind turbines and it has also been ascertained that the damage to the bearings is not because they are overloaded. The faults are inexplicable. We are continuing with measurements, but have decided to add an extra bearing, which will absorb the axle load on the gear box and be an effective way of preventing damage on all of the Valmet gear boxes," says Laurids Petersen. "If inspection of the gear box reveals any wear it will also be replaced and if there is the slightest doubt about the bearing, it will be changed too." The new gear boxes will have a five year guarantee. All the retrofit costs will be met by Vestas and Valmet.

The first four retrofit teams are already working on the problem and further teams are to be trained. All the work will be completed before the wind season starts in October.

New oil

NEG Micon has found a different solution. It will change the gear box oil in 240 of the company's 750 kW wind turbines at a total cost of DKK 10 million, says president Jens-Erik Kristensen. Inspections of the German gear boxes in the company's large wind turbines have not revealed faults, he adds. "Our gear supplier, Flender, together with oil experts at a German university, have discovered that the oil in the gear boxes was combining with water and that was why particles were being found in the oil; that's the way it's been explained to me." he says. "So we will get going with changing the oil in all our 750/48 turbines. Luckily this isn't a problem that can't be solved."

Kristensen says the 750 kW turbine is the only NEG Micon model which has had gear box problems, with the exception of a couple of 600 kW turbines. The gear box supplier, says Kristensen, has accepted responsibility for the oil problem and will pay the cost of the necessary oil changes. Unless a wind turbine is showing signs of a problem, the oil will changed as part of a routine maintenance visit. If it proves necessary to replace a component, for example an entire gear box, NEG Micon will supply the part with a two year guarantee, says Kristensen.

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